A large percentage of the electrical power generated in the United States of America is created in coal combustion power plants. The bulk of worldwide electricity production similarly relies on coal as a primary energy source. It is likely that coal will remain a primary energy source in the foreseeable future given the long term environmental concerns with the storage of waste from nuclear energy generation operations and the inefficiencies associated with solar powered electrical generation. In addition, vast worldwide coal reserves exist sufficient for at least 200 years of energy production at current rates.
However, there is and will continue to be a high demand to reduce the emissions of pollutants associated with coal-fired electrical energy generation and to increase the overall efficiency of the coal-fired generation process. Traditionally, in power plants and other industrial combustion settings, the efficiency of the combustion process and the level of pollution emission have been determined indirectly through measurements taken on extracted gas samples with techniques such as non-dispersive infrared (NDIR) photometry. Extractive sampling systems are not particularly well suited to closed loop control of a combustion process since a significant delay can be introduced between the time of gas extraction and the ultimate analysis. In addition, extractive processes generally result in a single point measurement which may or may not be representative of the actual concentration of the measured species within what can be a highly variable and dynamic combustion process chamber.
Laser-based optical molecular species sensors have recently been implemented to address the concerns associated with extraction measurement techniques. Laser-based measurement techniques can be implemented in situ and offer the further advantage of high-speed feedback suitable for dynamic process control. A particularly promising technique for measuring combustion gas composition, temperature and other combustion parameters is tunable diode laser absorption spectroscopy (TDLAS). TDLAS is well suited for the control and monitoring of coal-fired combustion processes. TDLAS is equally well suited for the monitoring of other combustion processes. In particular, the spectroscopy techniques described herein are useful for monitoring and controlling jet aircraft engine combustion parameters. TDLAS is typically implemented with diode lasers operating in the near-infrared and mid-infrared spectral regions. Suitable lasers have been extensively developed for use in the telecommunications industry and are, therefore, readily available for TDLAS applications. Various techniques of TDLAS which are more or less suitable for the sensing and control of combustion processes have been developed. Commonly known techniques are wavelength modulation spectroscopy, frequency modulation spectroscopy and direct absorption spectroscopy. Each of these techniques is based upon a predetermined relationship between the quantity and nature of laser light received by a detector after the light has been transmitted through a combustion process chamber and absorbed in specific spectral bands which are characteristic of the gases present in the process or combustion chamber. The absorption spectrum received by the detector is used to determine the quantity of the gas species under analysis plus associated combustion parameters such as temperature.
Typical coal-fired power plants have combustion chamber dimensions of 10-20 meters on a side. The plants are fired by pulverized coal, which results in a combustion process which both obscures the transmission of laser light because of the high dust load and which is extremely luminous. The environment is also highly turbulent. The overall transmission rate of light through the process chamber will fluctuate dramatically over time as a result of broadband absorption, scattering by particles or beam steering owing to refractive-index fluctuations. There is also intense thermal background radiation from the burning coal particles which can interfere with detector signals. The environment outside of the power plant boiler also makes the implementation of a TDLAS sensing or control system problematic. For example, any electronics, optics or other sensitive spectroscopy components must be positioned away from intense heat, or adequately shielded and cooled. Even though the implementation of a TDLAS system is extremely difficult under these conditions, TDLAS is particularly well suited to monitor and control a coal combustion process.
As discussed in detail in International Patent Application Serial Number PCT/US04/10048 (Publication Number WO 2004/090496), entitled METHOD AND APPARATUS FOR THE MONITORING AND CONTROL OF COMBUSTION, filed Mar. 31, 2004, which application is incorporated herein by reference in its entirety, optical fiber coupling is particularly advantageous for the implementation of a TDLAS system. In a fiber-coupled system, one or more probe beams which may consist of multiplexed light of various relevant wavelengths are delivered to a pitch-side (transmit) optical apparatus and projected into the combustion chamber. The probe beam is received in a catch-side (receive) optical apparatus after transversing the combustion chamber. As detailed in International Patent Application Serial Number PCT/US04/10048, it is advantageous to use a multimode optical fiber in the catch-side optical train. The use of a multimode fiber necessarily results in mode noise, which is a change in the signal level of detected light that results from non-uniform time and wavelength varying light distribution in the core of the multimode fiber used to collect and transport light. “Catch”-side mode noise can obscure absorption features which must be observed for effective TDLAS.
The phenomenon of mode noise is not limited to or caused by TDLAS implementations which feature a catch-side multimode optical fiber. On the contrary, mode noise will inevitably occur in any multimode optical fiber of substantial length which is transmitting light. Mode noise is inevitable in a multimode fiber because the greater cross sectional diameter of a multimode fiber as compared to a single-mode fiber allows transmitted light to propagate along numerous light paths or modes. Some paths or modes are longer or shorter than others. Thus, constructive and destructive interference necessarily occur resulting in non-uniform time and wavelength varying light distribution in the core of the multimode fiber which gives rise to a typical mode noise speckle pattern. Thus, mode noise occurs in computing, telecommunication, or other scientific applications utilizing substantial lengths of multimode fiber. Whether or not mode noise interferes with the efficiency of a given optical system depends upon the requirements of the particular system.
The present invention is directed toward overcoming one or more of the problems discussed above.